Interactions between decoder-side intra mode derivation and adaptive intra prediction modes

ABSTRACT

A method of performing intra prediction of a current block of a picture of a video sequence, includes determining whether a first flag indicates that an intra prediction mode corresponding to the current block is a directional mode, and based on the first flag being determined to indicate that the intra prediction mode corresponding to the current block is the directional mode, determining an index of the intra prediction mode in an allowed intra prediction modes (AIPM) list, and performing the intra prediction of the current block, using the intra prediction mode corresponding to the determined index in the AIPM list.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. application Ser.No. 17/094,923 filed Nov. 11, 2020, which claims priority from U.S.Provisional Patent Application No. 62/970,505, filed on Feb. 5, 2020, inthe U.S. Patent and Trademark Office, which is incorporated herein byreference in its entirety.

FIELD

Methods and apparatuses consistent with embodiments relate to videocoding, and more particularly, a method and an apparatus forinteractions between decoder-side intra mode derivation and adaptiveintra prediction modes.

BACKGROUND

The video coding format VP9 supports 8 directional modes correspondingto angles from 45 to 207 degrees. To exploit more varieties of spatialredundancy in directional textures, in the video coding format AOMediaVideo 1 (AV1), directional intra modes are extended to an angle set withfiner granularity. The original 8 angles are slightly changed and madeas nominal angles, and these 8 nominal angles are named V_PRED, H_PRED,D45_PRED, D135_PRED, D113_PRED, D157_PRED, D203_PRED, and D67_PRED, asillustrated in FIG. 1 . For each nominal angle, it has 7 finer angles,so AV1 has 56 directional angles in total. The prediction angle ispresented by a nominal intra angle plus an angle delta, which is −3˜3multiples of the step size of 3 degrees. To implement directionalprediction modes in AV1 via a generic way, all the 56 directional intraprediction modes in AV1 are implemented with a unified directionalpredictor that projects each pixel to a reference sub-pixel location andinterpolates the reference pixel by a 2-tap bilinear filter.

SUMMARY

According to embodiments, a method of performing intra prediction of acurrent block of a picture of a video sequence, is performed by at leastone processor, and includes determining whether a first flag indicatesthat an intra prediction mode corresponding to the current block is adirectional mode, and based on the first flag being determined toindicate that the intra prediction mode corresponding to the currentblock is the directional mode, determining an index of the intraprediction mode in an allowed intra prediction modes (AIPM) list, andperforming the intra prediction of the current block, using the intraprediction mode corresponding to the determined index in the AIPM list.

According to embodiments, an apparatus for performing intra predictionof a current block of a picture of a video sequence, includes at leastone memory configured to store computer program code, and at least oneprocessor configured to access the at least one memory and operateaccording to the computer program code. The computer program codeincludes first determining code configured to cause the at least oneprocessor to determine whether a first flag indicates that an intraprediction mode corresponding to the current block is a directionalmode, second determining code configured to cause the at least oneprocessor to, based on the first flag being determined to indicate thatthe intra prediction mode corresponding to the current block is thedirectional mode, determine an index of the intra prediction mode in anallowed intra prediction modes (AIPM) list, and first performing codeconfigured to cause the at least one processor to, based on the firstflag being determined to indicate that the intra prediction modecorresponding to the current block is the directional mode, perform theintra prediction of the current block, using the intra prediction modecorresponding to the determined index in the AIPM list.

According to embodiments, a non-transitory computer-readable storagemedium stores instructions that cause at least one processor todetermine whether a first flag indicates that an intra prediction modecorresponding to a current block is a directional mode, and based on thefirst flag being determined to indicate that the intra prediction modecorresponding to the current block is the directional mode, determine anindex of the intra prediction mode in an allowed intra prediction modes(AIPM) list, and perform intra prediction of the current block, usingthe intra prediction mode corresponding to the determined index in theAIPM list.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of eight nominal angles in AV1.

FIG. 2 is a simplified block diagram of a communication system accordingto embodiments.

FIG. 3 is a diagram of a placement of a video encoder and a videodecoder in a streaming environment, according to embodiments.

FIG. 4 is a functional block diagram of a video decoder according toembodiments.

FIG. 5 is a functional block diagram of a video encoder according toembodiments.

FIG. 6A is a diagram of top, left, and top-left positions for a PAETHmode.

FIG. 6B is a diagram of recursive intra filtering modes.

FIG. 6C is a diagram illustrating a template selection from areconstructed area with T lines of pixels.

FIG. 6D is a diagram illustrating a prediction fusion by weightedaveraging two Histogram of Gradient (HoG) modes and a planar mode.

FIG. 7 is a flowchart illustrating a method of performing intraprediction of a current block of a picture of a video sequence,according to embodiments.

FIG. 8 is a simplified block diagram of an apparatus for performingintra prediction of a current block of a picture of a video sequence,according to embodiments.

FIG. 9 is a diagram of a computer system suitable for implementingembodiments.

DETAILED DESCRIPTION

FIG. 2 is a simplified block diagram of a communication system (200)according to embodiments. The communication system (200) may include atleast two terminals (210-220) interconnected via a network (250). Forunidirectional transmission of data, a first terminal (210) may codevideo data at a local location for transmission to the other terminal(220) via the network (250). The second terminal (220) may receive thecoded video data of the other terminal from the network (250), decodethe coded data and display the recovered video data. Unidirectional datatransmission may be common in media serving applications and the like.

FIG. 2 illustrates a second pair of terminals (230, 240) provided tosupport bidirectional transmission of coded video that may occur, forexample, during videoconferencing. For bidirectional transmission ofdata, each terminal (230, 240) may code video data captured at a locallocation for transmission to the other terminal via the network (250).Each terminal (230, 240) also may receive the coded video datatransmitted by the other terminal, may decode the coded data and maydisplay the recovered video data at a local display device.

In FIG. 2 , the terminals (210-240) may be illustrated as servers,personal computers and smart phones but the principles of embodimentsare not so limited. Embodiments find application with laptop computers,tablet computers, media players and/or dedicated video conferencingequipment. The network (250) represents any number of networks thatconvey coded video data among the terminals (210-240), including forexample wireline and/or wireless communication networks. Thecommunication network (250) may exchange data in circuit-switched and/orpacket-switched channels. Representative networks includetelecommunications networks, local area networks, wide area networksand/or the Internet. For the purposes of the present discussion, thearchitecture and topology of the network (250) may be immaterial to theoperation of embodiments unless explained herein below.

FIG. 3 is a diagram of a placement of a video encoder and a videodecoder in a streaming environment, according to embodiments. Thedisclosed subject matter can be equally applicable to other videoenabled applications, including, for example, video conferencing,digital TV, storing of compressed video on digital media including CD,DVD, memory stick and the like, and so on.

A streaming system may include a capture subsystem (313) that caninclude a video source (301), for example a digital camera, creating,for example, an uncompressed video sample stream (302). That samplestream (302), depicted as a bold line to emphasize a high data volumewhen compared to encoded video bitstreams, can be processed by anencoder (303) coupled to the camera (301). The encoder (303) can includehardware, software, or a combination thereof to enable or implementaspects of the disclosed subject matter as described in more detailbelow. The encoded video bitstream (304), depicted as a thin line toemphasize the lower data volume when compared to the sample stream, canbe stored on a streaming server (305) for future use. One or morestreaming clients (306, 308) can access the streaming server (305) toretrieve copies (307, 309) of the encoded video bitstream (304). Aclient (306) can include a video decoder (310), which decodes theincoming copy of the encoded video bitstream (307) and creates anoutgoing video sample stream (311) that can be rendered on a display(312) or other rendering device (not depicted). In some streamingsystems, the video bitstreams (304, 307, 309) can be encoded accordingto certain video coding/compression standards. Examples of thosestandards include ITU-T Recommendation H.265. Under development is avideo coding standard informally known as VVC. The disclosed subjectmatter may be used in the context of VVC.

FIG. 4 is a functional block diagram of a video decoder (310) accordingto embodiments.

A receiver (410) may receive one or more codec video sequences to bedecoded by the decoder (310); in the same or embodiments, one codedvideo sequence at a time, where the decoding of each coded videosequence is independent from other coded video sequences. The codedvideo sequence may be received from a channel (412), which may be ahardware/software link to a storage device, which stores the encodedvideo data. The receiver (410) may receive the encoded video data withother data, for example, coded audio data and/or ancillary data streams,that may be forwarded to their respective using entities (not depicted).The receiver (410) may separate the coded video sequence from the otherdata. To combat network jitter, a buffer memory (415) may be coupled inbetween receiver (410) and entropy decoder/parser (420) (“parser”henceforth). When receiver (410) is receiving data from a store/forwarddevice of sufficient bandwidth and controllability, or from anisosychronous network, the buffer (415) may not be needed, or can besmall. For use on best effort packet networks such as the Internet, thebuffer (415) may be required, can be comparatively large and canadvantageously of adaptive size.

The video decoder (310) may include a parser (420) to reconstructsymbols (421) from the entropy coded video sequence. Categories of thosesymbols include information used to manage operation of the decoder(310), and potentially information to control a rendering device such asa display (312) that is not an integral part of the decoder but can becoupled to it, as was shown in FIG. 4 . The control information for therendering device(s) may be in the form of Supplementary EnhancementInformation (SEI messages) or Video Usability Information (VUI)parameter set fragments (not depicted). The parser (420) mayparse/entropy-decode the coded video sequence received. The coding ofthe coded video sequence can be in accordance with a video codingtechnology or standard, and can follow principles well known to a personskilled in the art, including variable length coding, Huffman coding,arithmetic coding with or without context sensitivity, and so forth. Theparser (420) may extract from the coded video sequence, a set ofsubgroup parameters for at least one of the subgroups of pixels in thevideo decoder, based upon at least one parameters corresponding to thegroup. Subgroups can include Groups of Pictures (GOPs), pictures, tiles,slices, macroblocks, Coding Units (CUs), blocks, Transform Units (TUs),Prediction Units (PUs) and so forth. The entropy decoder/parser may alsoextract from the coded video sequence information such as transformcoefficients, quantizer parameter (QP) values, motion vectors, and soforth.

The parser (420) may perform entropy decoding/parsing operation on thevideo sequence received from the buffer (415), so to create symbols(421). The parser (420) may receive encoded data, and selectively decodeparticular symbols (421). Further, the parser (420) may determinewhether the particular symbols (421) are to be provided to a MotionCompensation Prediction unit (453), a scaler/inverse transform unit(451), an Intra Prediction unit (452), or a loop filter unit (454).

Reconstruction of the symbols (421) can involve multiple different unitsdepending on the type of the coded video picture or parts thereof (suchas: inter and intra picture, inter and intra block), and other factors.Which units are involved, and how, can be controlled by the subgroupcontrol information that was parsed from the coded video sequence by theparser (420). The flow of such subgroup control information between theparser (420) and the multiple units below is not depicted for clarity.

Beyond the functional blocks already mentioned, decoder (310) can beconceptually subdivided into a number of functional units as describedbelow. In a practical implementation operating under commercialconstraints, many of these units interact closely with each other andcan, at least partly, be integrated into each other. However, for thepurpose of describing the disclosed subject matter, the conceptualsubdivision into the functional units below is appropriate.

A first unit is the scaler/inverse transform unit (451). Thescaler/inverse transform unit (451) receives quantized transformcoefficient as well as control information, including which transform touse, block size, quantization factor, quantization scaling matrices,etc. as symbol(s) (421) from the parser (420). It can output blocksincluding sample values that can be input into aggregator (455).

In some cases, the output samples of the scaler/inverse transform (451)can pertain to an intra coded block; that is: a block that is not usingpredictive information from previously reconstructed pictures, but canuse predictive information from previously reconstructed parts of thecurrent picture. Such predictive information can be provided by an intrapicture prediction unit (452). In some cases, the intra pictureprediction unit (452) generates a block of the same size and shape ofthe block under reconstruction, using surrounding already reconstructedinformation fetched from the current (partly reconstructed) picture(456). The aggregator (455), in some cases, adds, on a per sample basis,the prediction information the intra prediction unit (452) has generatedto the output sample information as provided by the scaler/inversetransform unit (451).

In other cases, the output samples of the scaler/inverse transform unit(451) can pertain to an inter coded, and potentially motion compensatedblock. In such a case, a Motion Compensation Prediction unit (453) canaccess reference picture memory (457) to fetch samples used forprediction. After motion compensating the fetched samples in accordancewith the symbols (421) pertaining to the block, these samples can beadded by the aggregator (455) to the output of the scaler/inversetransform unit (in this case called the residual samples or residualsignal) so to generate output sample information. The addresses withinthe reference picture memory form where the motion compensation unitfetches prediction samples can be controlled by motion vectors,available to the motion compensation unit in the form of symbols (421)that can have, for example X, Y, and reference picture components.Motion compensation also can include interpolation of sample values asfetched from the reference picture memory when sub-sample exact motionvectors are in use, motion vector prediction mechanisms, and so forth.

The output samples of the aggregator (455) can be subject to variousloop filtering techniques in the loop filter unit (454). Videocompression technologies can include in-loop filter technologies thatare controlled by parameters included in the coded video bitstream andmade available to the loop filter unit (454) as symbols (421) from theparser (420), but can also be responsive to meta-information obtainedduring the decoding of previous (in decoding order) parts of the codedpicture or coded video sequence, as well as responsive to previouslyreconstructed and loop-filtered sample values.

The output of the loop filter unit (454) can be a sample stream that canbe output to the render device (312) as well as stored in the referencepicture memory (456) for use in future inter-picture prediction.

Certain coded pictures, once fully reconstructed, can be used asreference pictures for future prediction. Once a coded picture is fullyreconstructed and the coded picture has been identified as a referencepicture (by, for example, parser (420)), the current reference picture(456) can become part of the reference picture buffer (457), and a freshcurrent picture memory can be reallocated before commencing thereconstruction of the following coded picture.

The video decoder (310) may perform decoding operations according to apredetermined video compression technology that may be documented in astandard, such as ITU-T Rec. H.265. The coded video sequence may conformto a syntax specified by the video compression technology or standardbeing used, in the sense that it adheres to the syntax of the videocompression technology or standard, as specified in the videocompression technology document or standard and specifically in theprofiles document therein. Also necessary for compliance can be that thecomplexity of the coded video sequence is within bounds as defined bythe level of the video compression technology or standard. In somecases, levels restrict the maximum picture size, maximum frame rate,maximum reconstruction sample rate (measured in, for example megasamplesper second), maximum reference picture size, and so on. Limits set bylevels can, in some cases, be further restricted through HypotheticalReference Decoder (HRD) specifications and metadata for HRD buffermanagement signaled in the coded video sequence.

In embodiments, the receiver (410) may receive additional (redundant)data with the encoded video. The additional data may be included as partof the coded video sequence(s). The additional data may be used by thevideo decoder (310) to properly decode the data and/or to moreaccurately reconstruct the original video data. Additional data can bein the form of, for example, temporal, spatial, or signal-to-noise ratio(SNR) enhancement layers, redundant slices, redundant pictures, forwarderror correction codes, and so on.

FIG. 5 is a functional block diagram of a video encoder (303) accordingto embodiments.

The encoder (303) may receive video samples from a video source (301)(that is not part of the encoder) that may capture video image(s) to becoded by the encoder (303).

The video source (301) may provide the source video sequence to be codedby the encoder (303) in the form of a digital video sample stream thatcan be of any suitable bit depth (for example: 8 bit, 10 bit, 12 bit, .. . ), any color space (for example, BT.601 Y CrCB, RGB, . . . ) and anysuitable sampling structure (for example Y CrCb 4:2:0, Y CrCb 4:4:4). Ina media serving system, the video source (301) may be a storage devicestoring previously prepared video. In a videoconferencing system, thevideo source (301) may be a camera that captures local image informationas a video sequence. Video data may be provided as a plurality ofindividual pictures that impart motion when viewed in sequence. Thepictures themselves may be organized as a spatial array of pixels,wherein each pixel can include one or more samples depending on thesampling structure, color space, etc. in use. A person skilled in theart can readily understand the relationship between pixels and samples.The description below focuses on samples.

According to embodiments, the encoder (303) may code and compress thepictures of the source video sequence into a coded video sequence (543)in real time or under any other time constraints as required by theapplication. Enforcing appropriate coding speed is one function ofController (550). Controller controls other functional units asdescribed below and is functionally coupled to these units. The couplingis not depicted for clarity. Parameters set by controller can includerate control related parameters (picture skip, quantizer, lambda valueof rate-distortion optimization techniques, . . . ), picture size, groupof pictures (GOP) layout, maximum motion vector search range, and soforth. A person skilled in the art can readily identify other functionsof controller (550) as they may pertain to video encoder (303) optimizedfor a certain system design.

Some video encoders operate in what a person skilled in the art readilyrecognizes as a “coding loop.” As an oversimplified description, acoding loop can consist of the encoding part of an encoder (530)(“source coder” henceforth) (responsible for creating symbols based onan input picture to be coded, and a reference picture(s)), and a (local)decoder (533) embedded in the encoder (303) that reconstructs thesymbols to create the sample data that a (remote) decoder also wouldcreate (as any compression between symbols and coded video bitstream islossless in the video compression technologies considered in thedisclosed subject matter). That reconstructed sample stream is input tothe reference picture memory (534). As the decoding of a symbol streamleads to bit-exact results independent of decoder location (local orremote), the reference picture buffer content is also bit exact betweenlocal encoder and remote encoder. In other words, the prediction part ofan encoder “sees” as reference picture samples exactly the same samplevalues as a decoder would “see” when using prediction during decoding.This fundamental principle of reference picture synchronicity (andresulting drift, if synchronicity cannot be maintained, for examplebecause of channel errors) is well known to a person skilled in the art.

The operation of the “local” decoder (533) can be the same as of a“remote” decoder (310), which has already been described in detail abovein conjunction with FIG. 4 . Briefly referring also to FIG. 4 , however,as symbols are available and en/decoding of symbols to a coded videosequence by entropy coder (545) and parser (420) can be lossless, theentropy decoding parts of decoder (310), including channel (412),receiver (410), buffer (415), and parser (420) may not be fullyimplemented in local decoder (533).

An observation that can be made at this point is that any decodertechnology except the parsing/entropy decoding that is present in adecoder also necessarily needs to be present, in substantially identicalfunctional form, in a corresponding encoder. The description of encodertechnologies can be abbreviated as they are the inverse of thecomprehensively described decoder technologies. Only in certain areas amore detail description is required and provided below.

As part of its operation, the source coder (530) may perform motioncompensated predictive coding, which codes an input frame predictivelywith reference to one or more previously-coded frames from the videosequence that were designated as “reference frames.” In this manner, thecoding engine (532) codes differences between pixel blocks of an inputframe and pixel blocks of reference frame(s) that may be selected asprediction reference(s) to the input frame.

The local video decoder (533) may decode coded video data of frames thatmay be designated as reference frames, based on symbols created by thesource coder (530). Operations of the coding engine (532) mayadvantageously be lossy processes. When the coded video data may bedecoded at a video decoder (not shown in FIG. 4 ), the reconstructedvideo sequence typically may be a replica of the source video sequencewith some errors. The local video decoder (533) replicates decodingprocesses that may be performed by the video decoder on reference framesand may cause reconstructed reference frames to be stored in thereference picture cache (534). In this manner, the encoder (303) maystore copies of reconstructed reference frames locally that have commoncontent as the reconstructed reference frames that will be obtained by afar-end video decoder (absent transmission errors).

The predictor (535) may perform prediction searches for the codingengine (532). That is, for a new frame to be coded, the predictor (535)may search the reference picture memory (534) for sample data (ascandidate reference pixel blocks) or certain metadata such as referencepicture motion vectors, block shapes, and so on, that may serve as anappropriate prediction reference for the new pictures. The predictor(535) may operate on a sample block-by-pixel block basis to findappropriate prediction references. In some cases, as determined bysearch results obtained by the predictor (535), an input picture mayhave prediction references drawn from multiple reference pictures storedin the reference picture memory (534).

The controller (550) may manage coding operations of the video coder(530), including, for example, setting of parameters and subgroupparameters used for encoding the video data.

Output of all aforementioned functional units may be subjected toentropy coding in the entropy coder (545). The entropy coder translatesthe symbols as generated by the various functional units into a codedvideo sequence, by loss-less compressing the symbols according totechnologies known to a person skilled in the art as, for exampleHuffman coding, variable length coding, arithmetic coding, and so forth.

The transmitter (540) may buffer the coded video sequence(s) as createdby the entropy coder (545) to prepare it for transmission via acommunication channel (560), which may be a hardware/software link to astorage device that may store the encoded video data. The transmitter(540) may merge coded video data from the video coder (530) with otherdata to be transmitted, for example, coded audio data and/or ancillarydata streams (sources not shown).

The controller (550) may manage operation of the encoder (303). Duringcoding, the controller (550) may assign to each coded picture a certaincoded picture type, which may affect the coding techniques that may beapplied to the respective picture. For example, pictures often may beassigned as one of the following frame types:

An Intra Picture (I picture) may be one that may be coded and decodedwithout using any other frame in the sequence as a source of prediction.Some video codecs allow for different types of Intra pictures,including, for example Independent Decoder Refresh Pictures. A personskilled in the art is aware of those variants of I pictures and theirrespective applications and features.

A Predictive picture (P picture) may be one that may be coded anddecoded using intra prediction or inter prediction using at most onemotion vector and reference index to predict the sample values of eachblock.

A Bi-directionally Predictive Picture (B Picture) may be one that may becoded and decoded using intra prediction or inter prediction using atmost two motion vectors and reference indices to predict the samplevalues of each block. Similarly, multiple-predictive pictures can usemore than two reference pictures and associated metadata for thereconstruction of a single block.

Source pictures commonly may be subdivided spatially into a plurality ofsample blocks (for example, blocks of 4×4, 8×8, 4×8, or 16×16 sampleseach) and coded on a block-by-block basis. Blocks may be codedpredictively with reference to other (already coded) blocks asdetermined by the coding assignment applied to the blocks' respectivepictures. For example, blocks of I pictures may be codednon-predictively or they may be coded predictively with reference toalready coded blocks of the same picture (spatial prediction or intraprediction). Pixel blocks of P pictures may be coded non-predictively,via spatial prediction or via temporal prediction with reference to onepreviously coded reference pictures. Blocks of B pictures may be codednon-predictively, via spatial prediction or via temporal prediction withreference to one or two previously coded reference pictures.

The video coder (303) may perform coding operations according to apredetermined video coding technology or standard, such as ITU-T Rec.H.265 or Versatile Video Coding (VVC) H.266. In its operation, the videocoder (303) may perform various compression operations, includingpredictive coding operations that exploit temporal and spatialredundancies in the input video sequence. The coded video data,therefore, may conform to a syntax specified by the video codingtechnology or standard being used.

In embodiments, the transmitter (540) may transmit additional data withthe encoded video. The video coder (530) may include such data as partof the coded video sequence. Additional data may include temporal,spatial, and/or SNR enhancement layers, other forms of redundant datasuch as redundant pictures and slices, Supplementary EnhancementInformation (SEI) messages, Visual Usability Information (VUI) parameterset fragments, and so on.

In AV1, there are 5 non-directional smooth intra prediction modes, whichare DC, PAETH, SMOOTH, SMOOTH_V, and SMOOTH_H. For DC prediction, theaverage of left and above neighboring samples is used as the predictorof the block to be predicted. For a PAETH predictor, top, left andtop-left reference samples are firstly fetched, and then the value thatis closest to (top+left−topleft) is set as the predictor for the pixelto be predicted. FIG. 6A illustrates the positions of top, left, andtop-left samples for one pixel in current block. For SMOOTH, SMOOTH_V,and SMOOTH_H modes, they predict the block using quadratic interpolationin vertical or horizontal directions, or the average of both directions.

To capture decaying spatial correlation with references on the edges,filter intra modes are designed for luma blocks. Five filter intra modesare defined for AV1, each represented by a set of eight 7-tap filtersreflecting a correlation between pixels in a 4×2 patch and 7 neighborsadjacent to it. In other words, the weighting factors for a 7-tap filterare position dependent. Take an 8×8 block for example; it is split into8 4×2 patches, which is shown in FIG. 6B. These patches are indicated byB0, B1, B2, B3, B4, B5, B6, and B7 in FIG. 6B. For each patch, its 7neighbors, indicated by R0˜R6, are used to predict the pixels in currentpatch. For patch B0, all the neighbors are already reconstructed. Butfor other patches, not all the neighbors are reconstructed, then thepredicted values of immediate neighbors are used as the references. Forexample, all the neighbors of patch B7 are not reconstructed, so theprediction samples of neighbors (i.e., B5 and B6) are used instead.

Chroma from Luma (CfL) is a chroma-only intra predictor that modelschroma pixels as a linear function of coincident reconstructed lumapixels. The CfL prediction is expressed as follows:CfL(α)=α×L _(AC) +DC  (1),

wherein L_(AC) denotes the AC contribution of luma component, a denotesthe parameter of the linear model, and DC denotes the DC contribution ofthe chroma component. To be specific, the reconstructed luma pixels aresubsampled into the chroma resolution, and then the average value issubtracted to form the AC contribution. To approximate chroma ACcomponent from the AC contribution, instead of requiring the decoder tocalculate the scaling parameters as in some method, AV1 CfL determinesthe parameter a based on the original chroma pixels and signals them inthe bitstream. This reduces decoder complexity and yields more precisepredictions. As for the DC contribution of the chroma component, it iscomputed using intra DC mode, which is sufficient for most chromacontent and has mature fast implementations.

Proposals were made to improve the intra mode coding for the VersatileVideo Coding (VVC) standard. For example, two intra prediction mode setsmay be defined for each block, which are named as an allowed intraprediction mode set (AIPM, also named as adaptive intra predictionmodes) and a disallowed intra prediction mode (DIPM) set. AIPM isdefined as one mode set with modes that can be used for intra predictionof a current block, and DIPM is defined as one mode set with modes thatcannot be signaled or used for intra prediction of the current block.For each block, the modes in these two mode sets are derived accordingto the intra prediction modes of neighboring blocks. Neighboring modesare included in the AIPM set but not included in the DIPM set. Thenumber of modes included in the AIPM and DIPM sets are predefined andfixed for all blocks. When the size of the AIPM set is S and the numberof derived intra prediction modes from neighboring modes are less thanS, the default modes are used to fill the AIPM set.

When applying AIPM to AV1, all the nominal angles are always included inthe AIPM despite the block size of a current block and prediction modesof neighboring blocks.

In a decoder-side intra mode derivation (DIMD) process, an intraprediction mode is derived based on previously encoded/decoded pixels,and this is done in the same way at the encoder and decoder sides.Therefore, in the DIMD process, the signaling of an intra predictionmode index is avoided. This process defines a new coding mode calledDIMD. One flag is signaled in the bitstream to indicate whether the DIMDmode is selected or not. A decoder-side intra mode derivation may bealso called Derived Intra Mode, which has been implemented in proposalsunder the flag CONFIG_DERIVED_INTRA_MODE.

Two main steps are employed in a DIMD process, which is described indetail as follows.

To implicitly derive an intra prediction mode (IPM) of a DIMD blocks, atexture gradient analysis is performed at both encoder and decodersides. This process starts with an empty HoG with 65 entries,corresponding to the number of angular modes. Amplitudes of theseentries are determined during the texture gradient analysis.

In the first step, DIMD picks a template of T=3 columns and rows fromrespectively left and above a current block, as shown in portion (a) ofFIG. 6C. This area will be used as the reference for the gradient-basedIPM derivation.

In the second step, the horizontal and vertical Sobel filters areapplied on all 3×3 window positions, centered on the pixels of themiddle line of the template, as shown in portion (b) of FIG. 6C. On eachwindow position, Sobel filters calculate the intensity of purehorizontal and vertical directions as G_hor and G_ver, respectively.Then, the texture angle of the window is calculated as:angle=arctan(G _(hor) /G _(hor))  (2),

which can be converted into one of 65 angular IPMs. Once the IPM indexof the current window is derived as idx, the amplitude of its entry inthe HoG[idx] is updated by the addition of:ampl=|G _(hor) |+|G _(hor)|  (3).

Portion (c) of FIG. 6C shows an example of a HoG calculated afterapplying the above operations on all pixel positions in the template.

If only one single IPM corresponding to the tallest spike of HoG wasused, this process is not needed.

Otherwise, if more than one IPM is derived from the DIMD process, thisprediction fusion process may be used.

Prediction fusion is computed by using the weighted average of multiplepredictors. FIG. 6D demonstrates one example of a fusion algorithm. Ascan be seen, two IPMs corresponding to the three tallest spikes of theHoG are detected as M1 and M2. The third IPM is fixed as a planar mode.After applying the pixel prediction by these three IPMs and obtainingPred1, Pred2 and Pred3, their fusion is computed by a weighted averageof the above three predictors. In one example, the weight of the planarmode is fixed to 21/64 (˜⅓). The remaining weight of 43/64 (˜⅔) is thenshared between the two HoG IPMs, proportionally to the amplitude oftheir HoG bars.

In detail, a first weight ω₁, a second weight ω₂, and a third weight ω₃may be expressed as follows:

$\begin{matrix}{{\omega_{1} = {\frac{43}{64} \times \frac{{ampl}( M_{1} )}{{{ampl}( M_{1} )} + {{ampl}( M_{2} )}}}};} & (4) \\{{\omega_{2} = {\frac{43}{64} \times \frac{{ampl}( M_{2} )}{{{ampl}( M_{1} )} + {{ampl}( M_{2} )}}}};{and}} & (5) \\{\omega_{3} = {\frac{21}{64}.}} & (6)\end{matrix}$

Accordingly, a predictor block may be expressed as follows:Σ_(i=1) ³ω_(i)×Pred_(i)  (7).

DIMD uses neighboring samples of a current block to derive one ormultiple angular IPMs and assign a shorter codeword for these derivedIPMs. AIPM uses the IPMs of neighboring modes to derive a selected IPMlist and assign a shorter codeword to the IPMs of neighboring modes.Both methods use the neighboring information to optimize the signalingof IPMs for the current block. However, there is no solution on how tocombine these two methods together.

Embodiments of a method and an apparatus for interactions betweendecoder-side intra mode derivation and adaptive intra prediction modes,are described herein.

In this detailed description, if one mode is not a smooth mode, or isgenerating prediction samples according to a given prediction direction,this one mode is called an angular mode or a directional mode. DIMD is ageneral term, and one process is called DIMD if it uses a neighboringreconstructed sample to derive an intra prediction mode.

In embodiments, there are two intra prediction mode sets for each block,which are named an AIPM set and a DIPM set. All non-directional modesare always included in the AIPM set despite a block size of a currentblock and prediction modes of neighboring blocks.

In an embodiment, all non-directional smooth intra prediction modes inAV1 are always firstly inserted into an AIPM set despite intraprediction modes of neighboring blocks.

In an embodiment, DC, PAETH, SMOOTH, SMOOTH_V, and SMOOTH_H modes arealways firstly included in an AIPM set despite intra prediction modes ofneighboring blocks.

In an embodiment, modes included in an AIPM set can be split into Klevels, K being a positive integer, such as 2 or 3 or 4. For a firstlevel, a number of modes is equal to a number of non-directional modes.For other levels, a number of modes is equal to a power of 2, such as2^(L), L being a positive integer larger than 1. For example, a numberof modes in an AIPM set may be S, and the AIPM set may have 3 levels. Sis equal to K+2^(L)+2^(M), wherein modes with an index smaller than K inthe AIPM set are called first level modes, and modes with an index equalto or larger than K but smaller than K+2^(L) in the AIPM set are calledsecond level modes, and so on. In an embodiment, all non-directionalIPMs are placed in a first level of an AIPM set.

In embodiments, only directional IPMs are included in an AIPM list, anda number of modes in the AIPM is set equal to a power of 2 or a sum ofmultiples of a power of 2.

In an embodiment, for signaling of intra prediction modes, one flag issignaled to indicate whether a current block is a directional mode ornot. If yes, a second flag is signaled to indicate an index of a currentmode in an AIPM list. Otherwise, the second flag is signaled to indicatewhich non-directional mode the current mode is.

In an embodiment, for the signaling of intra prediction modes, one flagis signaled to indicate whether a current block is directional mode ornot. If the current block is a directional mode, a second flag issignaled to indicate whether the current mode is a DIMD mode or not. Ifthe current mode is not a DIMD mode, then a third flag is signaled toindicate an index of the current mode in an AIPM list. Otherwise if thecurrent mode is a DIMD mode, then the third flag is avoided, and an IPMof the current block is derived from a decoder side. Otherwise if thecurrent mode is not a directional IPM, then the second flag is signaledto indicate which non-directional mode the current mode is.

In an embodiment, IPMs derived from a DIMD process are always insertedinto an AIPM list. In an embodiment, the IPMs derived from the DIMDprocess are always firstly inserted into the AIPM list and placed in afirst level of the AIPM list.

In embodiments, an AIPM scheme is applied to a luma component only,while a DIMD scheme is applied to a chroma component only.

FIG. 7 is a flowchart illustrating a method (700) of performing intraprediction of a current block of a picture of a video sequence,according to embodiments. In some implementations, one or more processblocks of FIG. 7 may be performed by the decoder (310). In someimplementations, one or more process blocks of FIG. 7 may be performedby another device or a group of devices separate from or including thedecoder (310), such as the encoder (303).

Referring to FIG. 7 , in a first block (710), the method (700) includesdetermining whether a first flag indicates that an intra prediction modecorresponding to the current block is a directional mode.

Based on the first flag being determined to indicate that the intraprediction mode corresponding to the current block is the directionalmode (710—Yes), in a second block (720), the method (700) includesdetermining whether a second flag indicates that the intra predictionmode is a decoder-side intra mode derivation (DIMD) mode.

Based on the second flag being determined to not indicate that the intraprediction mode corresponding to the current block is the DIMD mode(720—No), in a third block (730), the method (700) includes determiningan index of the intra prediction mode in an allowed intra predictionmodes (AIPM) list, and in a fourth block (740), the method (700)includes performing the intra prediction of the current block, using theintra prediction mode corresponding to the determined index in the AIPMlist.

Based on the second flag being determined to indicate that the intraprediction mode corresponding to the current block is the DIMD mode(720—Yes), in a fifth block (750), the method (700) includes performinga DIMD to determine the intra prediction mode, and continues in thefourth block (740), in which the method (700) includes performing theintra prediction of the current block, using the determined intraprediction mode.

Based on the first flag being determined to not indicate that the intraprediction mode corresponding to the current block is the directionalmode (710-N), in a sixth block (760), the method (700) includesdetermining the intra prediction mode to be one among non-directionalmodes, and continues to the fourth block (740), in which the method(700) includes performing the intra prediction of the current block,using the determined intra prediction mode.

The method (700) may further include inserting the intra prediction modedetermined by performing the DIMD, first into a first level of the AIPMlist.

The method (700) may further include performing intra prediction of aluma component of the current block, using at least one intra predictionmode in the AIPM list.

The method (700) may further include performing intra prediction of achroma component of the current block, using the intra prediction modedetermined by performing the DIMD.

The AIPM list may include only directional modes, and a number of thedirectional modes included in the AIPM list may be equal to a power of 2or a sum of multiples of the power of 2.

Although FIG. 7 shows example blocks of the method (700), in someimplementations, the method (700) may include additional blocks, fewerblocks, different blocks, or differently arranged blocks than thosedepicted in FIG. 7 . Additionally, or alternatively, two or more of theblocks of the method (700) may be performed in parallel.

FIG. 8 is a simplified block diagram of an apparatus (800) forperforming intra prediction of a current block of a picture of a videosequence, according to embodiments.

Referring to FIG. 8 , the apparatus (800) includes first determiningcode (805), second determining code (810), third determining code (815),first performing code (820), second performing code (825) and fourthdetermining code (830).

The first determining code (805) is configured to cause at least oneprocessor to determine whether a first flag indicates that an intraprediction mode corresponding to the current block is a directionalmode.

The third determining code (815) is configured to cause the at least oneprocessor to, based on the first flag being determined to indicate thatthe intra prediction mode corresponding to the current block is thedirectional mode, determine whether a second flag indicates that theintra prediction mode is a decoder-side intra mode derivation (DIMD)mode.

The second determining code (810) is configured to cause the at leastone processor to, based on the second flag being determined to notindicate that the intra prediction mode corresponding to the currentblock is the DIMD mode, determine an index of the intra prediction modein an allowed intra prediction modes (AIPM) list.

The first performing code (820) is configured to cause the at least oneprocessor to, based on the second flag being determined to not indicatethat the intra prediction mode corresponding to the current block is theDIMD mode, perform the intra prediction of the current block, using theintra prediction mode corresponding to the determined index in the AIPMlist.

The second performing code (825) is configured to cause the at least oneprocessor to, based on the second flag being determined to indicate thatthe intra prediction mode corresponding to the current block is the DIMDmode, perform a DIMD to determine the intra prediction mode.

The first performing code (820) is further configured to cause the atleast one processor to, based on the second flag being determined toindicate that the intra prediction mode corresponding to the currentblock is the DIMD mode, perform the intra prediction of the currentblock, using the determined intra prediction mode.

The fourth determining code (830) is configured to cause the at leastone processor to, based on the first flag being determined to notindicate that the intra prediction mode corresponding to the currentblock is the directional mode, determine the intra prediction mode to beone among non-directional modes.

The first performing code (820) is further configured to cause the atleast one processor to, based on the first flag being determined to notindicate that the intra prediction mode corresponding to the currentblock is the directional mode, perform the intra prediction of thecurrent block, using the determined intra prediction mode.

The apparatus (800) may further include inserting code configured tocause the at least one processor to insert the intra prediction modedetermined by performing the DWID, first into a first level of the AIPMlist.

The apparatus (800) may further include third performing code configuredto cause the at least one processor to perform intra prediction of aluma component of the current block, using at least one intra predictionmode in the AIPM list.

The third performing code may be further configured to cause the atleast one processor to perform intra prediction of a chroma component ofthe current block, using the intra prediction mode determined byperforming the DIMD.

The AIPM list may include only directional modes, and a number of thedirectional modes included in the AIPM list may be equal to a power of 2or a sum of multiples of the power of 2.

FIG. 9 is a diagram of a computer system (900) suitable for implementingembodiments.

The computer software can be coded using any suitable machine code orcomputer language, that may be subject to assembly, compilation,linking, or like mechanisms to create code including instructions thatcan be executed directly, or through interpretation, micro-codeexecution, and the like, by computer central processing units (CPUs),Graphics Processing Units (GPUs), and the like.

The instructions can be executed on various types of computers orcomponents thereof, including, for example, personal computers, tabletcomputers, servers, smartphones, gaming devices, internet of thingsdevices, and the like.

The components shown in FIG. 9 for computer system (900) are examples innature and are not intended to suggest any limitation as to the scope ofuse or functionality of the computer software implementing embodiments.Neither should the configuration of components be interpreted as havingany dependency or requirement relating to any one or combination ofcomponents illustrated in embodiments of a computer system (900).

Computer system (900) may include certain human interface input devices.Such a human interface input device may be responsive to input by one ormore human users through, for example, tactile input (such as:keystrokes, swipes, data glove movements), audio input (such as: voice,clapping), visual input (such as: gestures), olfactory input (notdepicted). The human interface devices can also be used to capturecertain media not necessarily directly related to conscious input by ahuman, such as audio (such as: speech, music, ambient sound), images(such as: scanned images, photographic images obtain from a still imagecamera), video (such as two-dimensional video, three-dimensional videoincluding stereoscopic video).

Input human interface devices may include one or more of (only one ofeach depicted): keyboard (901), mouse (902), trackpad (903), touchscreen (910), data-glove, joystick (905), microphone (906), scanner(907), camera (908).

Computer system (900) may also include certain human interface outputdevices. Such human interface output devices may be stimulating thesenses of one or more human users through, for example, tactile output,sound, light, and smell/taste. Such human interface output devices mayinclude tactile output devices (for example tactile feedback by thetouch-screen (910), data-glove, or joystick (905), but there can also betactile feedback devices that do not serve as input devices), audiooutput devices (such as: speakers (909), headphones (not depicted)),visual output devices (such as screens (910) to include cathode ray tube(CRT) screens, liquid-crystal display (LCD) screens, plasma screens,organic light-emitting diode (OLED) screens, each with or withouttouch-screen input capability, each with or without tactile feedbackcapability—some of which may be capable to output two dimensional visualoutput or more than three dimensional output through means such asstereographic output; virtual-reality glasses (not depicted),holographic displays and smoke tanks (not depicted)), and printers (notdepicted). A graphics adapter (950) generates and outputs images to thetouch-screen (910).

Computer system (900) can also include human accessible storage devicesand their associated media such as optical media including CD/DVD ROM/RW(920) with CD/DVD or the like media (921), thumb-drive (922), removablehard drive or solid state drive (923), legacy magnetic media such astape and floppy disc (not depicted), specialized ROM/ASIC/PLD baseddevices such as security dongles (not depicted), and the like.

Those skilled in the art should also understand that term “computerreadable media” as used in connection with the presently disclosedsubject matter does not encompass transmission media, carrier waves, orother transitory signals.

Computer system (900) can also include interface(s) to one or morecommunication networks (955). Networks (955) can for example bewireless, wireline, optical. Networks (955) can further be local,wide-area, metropolitan, vehicular and industrial, real-time,delay-tolerant, and so on. Examples of networks (955) include local areanetworks such as Ethernet, wireless LANs, cellular networks to includeglobal systems for mobile communications (GSM), third generation (3G),fourth generation (4G), fifth generation (5G), Long-Term Evolution(LTE), and the like, TV wireline or wireless wide area digital networksto include cable TV, satellite TV, and terrestrial broadcast TV,vehicular and industrial to include CANBus, and so forth. Certainnetworks (955) commonly require external network interface adapters thatattached to certain general purpose data ports or peripheral buses((949)) (such as, for example universal serial bus (USB) ports of thecomputer system (900); others are commonly integrated into the core ofthe computer system (900) by attachment to a system bus as describedbelow (for example Ethernet interface into a PC computer system orcellular network interface (954) into a smartphone computer system).Using any of these networks (955), computer system (900) can communicatewith other entities. Such communication can be uni-directional, receiveonly (for example, broadcast TV), uni-directional send-only (for exampleCANbus to certain CANbus devices), or bi-directional, for example toother computer systems using local or wide area digital networks.Certain protocols and protocol stacks can be used on each of thosenetworks (955) and network interfaces (954) as described above.

Aforementioned human interface devices, human-accessible storagedevices, and network interfaces (954) can be attached to a core (940) ofthe computer system (900).

The core (940) can include one or more Central Processing Units (CPU)(941), Graphics Processing Units (GPU) (942), specialized programmableprocessing units in the form of Field Programmable Gate Areas (FPGA)(943), hardware accelerators (944) for certain tasks, and so forth.These devices, along with Read-only memory (ROM) (945), Random-accessmemory (RAM) (946), internal mass storage such as internal non-useraccessible hard drives, solid-state drives (SSDs), and the like (947),may be connected through a system bus (948). In some computer systems,the system bus (948) can be accessible in the form of one or morephysical plugs to enable extensions by additional CPUs, GPU, and thelike. The peripheral devices can be attached either directly to thecore's system bus (948), or through a peripheral bus (949).Architectures for a peripheral bus include peripheral componentinterconnect (PCI), USB, and the like.

CPUs (941), GPUs (942), FPGAs (943), and accelerators (944) can executecertain instructions that, in combination, can make up theaforementioned computer code. That computer code can be stored in ROM(945) or RAM (946). Transitional data can also be stored in RAM (946),whereas permanent data can be stored for example, in the internal massstorage (947). Fast storage and retrieve to any of the memory devicescan be enabled through the use of cache memory, that can be closelyassociated with one or more CPU (941), GPU (942), mass storage (947),ROM (945), RAM (946), and the like.

The computer readable media can have computer code thereon forperforming various computer-implemented operations. The media andcomputer code can be those specially designed and constructed for thepurposes of embodiments, or they can be of the kind well known andavailable to those having skill in the computer software arts.

As an example and not by way of limitation, the computer system havingarchitecture (900), and specifically the core (940) can providefunctionality as a result of processor(s) (including CPUs, GPUs, FPGA,accelerators, and the like) executing software embodied in one or moretangible, computer-readable media. Such computer-readable media can bemedia associated with user-accessible mass storage as introduced above,as well as certain storage of the core (940) that are of non-transitorynature, such as core-internal mass storage (947) or ROM (945). Thesoftware implementing various embodiments can be stored in such devicesand executed by core (940). A computer-readable medium can include oneor more memory devices or chips, according to particular needs. Thesoftware can cause the core (940) and specifically the processorstherein (including CPU, GPU, FPGA, and the like) to execute particularprocesses or particular parts of particular processes described herein,including defining data structures stored in RAM (946) and modifyingsuch data structures according to the processes defined by the software.In addition or as an alternative, the computer system can providefunctionality as a result of logic hardwired or otherwise embodied in acircuit (for example: accelerator (944)), which can operate in place ofor together with software to execute particular processes or particularparts of particular processes described herein. Reference to softwarecan encompass logic, and vice versa, where appropriate. Reference to acomputer-readable media can encompass a circuit (such as an integratedcircuit (IC)) storing software for execution, a circuit embodying logicfor execution, or both, where appropriate. Embodiments encompass anysuitable combination of hardware and software.

While this disclosure has described several embodiments, there arealterations, permutations, and various substitute equivalents, whichfall within the scope of the disclosure. It will thus be appreciatedthat those skilled in the art will be able to devise numerous systemsand methods that, although not explicitly shown or described herein,embody the principles of the disclosure and are thus within the spiritand scope thereof.

The invention claimed is:
 1. A method of performing intra prediction ofa current block of a picture of a video sequence, the method beingperformed by at least one processor, and the method comprising:determining whether a first flag indicates that an intra prediction modecorresponding to the current block is a directional mode; based on thefirst flag being determined to indicate that the intra prediction modecorresponding to the current block is the directional mode, determiningwhether a second flag indicates that the intra prediction mode is adecoder side intra mode derivation (DIMD) mode; and based on the secondflag being determined to not indicate that the intra prediction mode isthe DIMD mode: signaling a third flag indicating an index of the intraprediction mode in an allowed intra prediction modes (AIPM) list; andperforming the intra prediction of the current block, using the intraprediction mode corresponding to the signaled third flag.
 2. The methodof claim 1, further comprising: based on the second flag beingdetermined to indicate that the intra prediction mode is in the DIMDmode: performing a DIMD to determine the intra prediction mode; andperforming the intra prediction of the current block, using thedetermined intra prediction mode.
 3. The method of claim 1, furthercomprising inserting the intra prediction mode determined by performinga decoder side intra mode derivation (DIMD), firstly into a first levelof the AIPM list.
 4. The method of claim 2, further comprisingperforming intra prediction of a chroma component of the current block,using the intra prediction mode determined by performing the DIMD. 5.The method of claim 1, further comprising, based on the first flag beingdetermined to not indicate that the intra prediction mode correspondingto the current block is the directional mode: determining the intraprediction mode to be one among non-directional modes; and performingthe intra prediction of the current block, using the determined intraprediction mode.
 6. The method of claim 1, wherein the performing theintra prediction of the current block, using the intra prediction modecorresponding to the signaled third flag comprises performing intraprediction of a luma component of the current block, using at least oneintra prediction mode in the AIPM list.
 7. The method of claim 1,wherein the AIPM list comprises only directional modes, and a number ofthe directional modes included in the AIPM list is equal to a power of 2or a sum of multiples of the power of
 2. 8. An apparatus for performingintra prediction of a current block of a picture of a video sequence,the apparatus comprising: at least one memory configured to storecomputer program code; and at least one processor configured to accessthe at least one memory and operate according to the computer programcode, the computer program code comprising: first determining codeconfigured to cause the at least one processor to determine whether afirst flag indicates that an intra prediction mode corresponding to thecurrent block is a directional mode; second determining code configuredto cause the at least one processor to, based on the first flag beingdetermined to indicate that the intra prediction mode corresponding tothe current block is the directional mode, determine whether a secondflag indicates that the intra prediction mode is a decoder side intramode derivation (DIMD) mode; and first performing code configured tocause the at least one processor to, based on the second flag beingdetermined to not indicate that the intra prediction mode is the DIMDmode, signal a third flag indicating an index of the intra predictionmode in an allowed intra prediction modes (AIPM) list and perform theintra prediction of the current block, using the intra prediction modecorresponding to the signaled third flag.
 9. The apparatus of claim 8,wherein the computer program code further comprises: second performingcode configured to cause the at least one processor to, based on thesecond flag being determined to indicate that the intra prediction modeis in the DIMD mode, perform a DIMD to determine the intra predictionmode, wherein the first performing code is further configured to causethe at least one processor to, based on the second flag being determinedto indicate that the intra prediction mode is in the DIMD mode, performthe intra prediction of the current block, using the determined intraprediction mode.
 10. The apparatus of claim 8, wherein the computerprogram code further comprises inserting code configured to cause the atleast one processor to insert the intra prediction mode determined byperforming a decoder side intra mode derivation (DIMD), firstly into afirst level of the AIPM list.
 11. The apparatus of claim 9, wherein thesecond performing code is further configured to cause the at least oneprocessor to perform the intra prediction of a chroma component of thecurrent block, using the intra prediction mode determined by performingthe DIMD.
 12. The apparatus of claim 8, wherein the computer programcode further comprises third determining code configured to cause the atleast one processor to, based on the first flag being determined to notindicate that the intra prediction mode corresponding to the currentblock is the directional mode, determine the intra prediction mode to beone among non-directional modes, and wherein the first performing codeis further configured to cause the at least one processor to, based onthe first flag being determined to not indicate that the intraprediction mode corresponding to the current block is the directionalmode, perform the intra prediction of the current block, using thedetermined intra prediction mode.
 13. The apparatus of claim 8, whereinthe first performing code is further configured to cause the at leastone processor to perform intra prediction of a luma component of thecurrent block, using at least one intra prediction mode in the AIPMlist.
 14. The apparatus of claim 8, wherein the AIPM list comprises onlydirectional modes, and a number of the directional modes included in theAIPM list is equal to a power of 2 or a sum of multiples of the power of2.
 15. A non-transitory computer-readable storage medium storinginstructions that cause at least one processor to: determine whether afirst flag indicates that an intra prediction mode corresponding to acurrent block is a directional mode; and based on the first flag beingdetermined to indicate that the intra prediction mode corresponding tothe current block is the directional mode, determining whether a secondflag indicates that the intra prediction mode is a decoder side intramode derivation (DIMD) mode; and based on the second flag beingdetermined to not indicate that the intra prediction mode is the DIMDmode: signaling a third flag indicating an index of the intra predictionmode in an allowed intra prediction modes (AIPM) list; and performingthe intra prediction of the current block, using the intra predictionmode corresponding to the signaled third flag.
 16. The non-transitorycomputer-readable storage medium of claim 15, wherein the instructionsfurther cause the at least one processor to: based on the second flagbeing determined to indicate that the intra prediction mode is in theDIMD mode: perform a DIMD to determine the intra prediction mode; andperform the intra prediction of the current block, using the determinedintra prediction mode.
 17. The non-transitory computer-readable storagemedium of claim 15, wherein the instructions further cause the at leastone processor to insert the intra prediction mode determined byperforming a decoder side intra mode derivation (DIMD), firstly into afirst level of the AIPM list.
 18. The non-transitory computer-readablestorage medium of claim 16, wherein the instructions further cause theat least one processor to perform intra prediction of a chroma componentof the current block, using the intra prediction mode determined byperforming the DIMD.
 19. The non-transitory computer-readable storagemedium of claim 15, wherein the instructions further cause the at leastone processor to, based on the first flag being determined to notindicate that the intra prediction mode corresponding to the currentblock is the directional mode: determine the intra prediction mode to beone among non-directional modes; and perform the intra prediction of thecurrent block, using the determined intra prediction mode.
 20. Thenon-transitory computer-readable storage medium of claim 15, wherein theinstructions further cause the at least one processor to perform intraprediction of a luma component of the current block, using at least oneintra prediction mode in the AIPM list.